In a cellular system represented by a cellular phone, a cell refers to a range in which a radio base station apparatus (hereinafter, simply referred to as a base station (BS)) can communicate with a radio terminal apparatus (hereinafter, simply referred to as a mobile station (MS) or user equipment (UE)), that is, a range (communication area) that the base station covers. In the cellular system, the communication area is extensively expanded by disposing a plurality of base stations.
In the cellular system, the same frequencies are repeatedly used between the cells to efficiently utilize finite frequency resources. For example, in the cellular system, a system band is divided into N frequency bands, and the N frequency bands are used in such a manner that the same frequency is not used by N adjacent cells (N-cell repetition use).
In the N-cell repetition use, the larger N is, the larger the distance between cells using the same frequency is. Therefore, an influence of inter-cell interference (ICI) between adjacent cells becomes smaller. However, the larger N is, the smaller the frequency bandwidth (that is, system bandwidth/N) usable in one cell is. That is, the magnitude of the ICI and the frequency bandwidth usable in one cell are in a trade-off relation.
For example, in order to build a broadband system such as the long term evolution (LTE) which is being standardized in a standard organization, 3rd generation partnership project (3GPP), it is desirable to use one-cell repetition use (that is, N=1) for which the frequency bandwidth usable in one cell is the maximum. That is, in the LTE, the ICI has to be reduced to use the same frequency band (system band) among all cells.
In the cellular system, typically, the reception power of a signal transmitted from a base station is attenuated for a terminal distant from a base station located at the center of the cell. That is, the reception power of the signal transmitted from the base station is increased for a terminal located near the center of the cell, whereas the reception power of the signal is decreased for a terminal located near the edge of the cell. On the other hand, the interference from an adjacent cell is decreased for a terminal (a terminal distant from the adjacent cell) located near the center of the cell, whereas the interference is increased for a terminal (a terminal close to the adjacent cell) located near the edge of the cell. That is, the influence of the ICI is decreased for the terminal located near the center of the cell, whereas the influence of the ICI is increased for the terminal located near the edge of the cell.
Accordingly, a coordinated multipoint transmission (CoMP transmission) technique has been studied as a technique of improving the reception characteristics (such as the throughput) of the terminal (the terminal located near the edge of the cell) largely affected by the ICI, (for example, see Patent Literature 1, Non-patent Literature 1, and Non-patent Literature 2). In the coordinated multipoint transmission, a plurality of adjacent base stations use the same frequency and coordinate to transmit signals to the same terminal. In the coordinated multipoint transmission, an inter-base-station cooperation MIMO communication scheme, a site diversity scheme, and the like have been suggested.
There has been suggested a conventional technique of improving the transmission capacity characteristics by using a hybrid scheme of a plural base station cooperation MIMO communication scheme using the CoMP transmission (hereinafter, referred to as CoMP transmission) and a single base station MIMO communication scheme (hereinafter, referred to as a single base station transmission) by a single base station (for example, see Non-Patent Literature 1). In the conventional technique, the single base station transmission is applied to a terminal (for example, a terminal located near the center of the cell) having satisfactory reception characteristics and the CoMP transmission is applied to a terminal (a terminal located near the edge of the cell) being largely affected by ICI and thus having unsatisfactory reception characteristics.